Clockwise from the top: A chameleon, an aye aye, a fossa and a baobab. All are native to Madagascar.

In celebration of the Year of Science's October theme, the geosciences and planet Earth, this month's story focuses on how geography and geology have shaped the evolution of life in one of Earth's most unique places. Madagascar, the fourth-largest island in the world, sits in the Indian Ocean several hundred kilometers off Africa's southeastern coast and provides a home to a remarkable variety of plant and animal species, including the aye aye, fossa, chameleon, and baobab tree. Madagascar has made the news lately because of a military-backed coup, which has threatened conservation efforts. Criminal groups have taken advantage of the political turmoil to illegally log rare trees in Madagascar's dwindling forests, and the future of the former president's plans to massively expand the island's conservation areas now seem uncertain.

Where's the evolution?
Conservationists are concerned about Madagascar because of the uniqueness of its biota, most of which is endemic  i.e., found nowhere else on Earth. Why is Madagascar home to so many unique plants and animals? Because the island's geography, geology, and climate have provided opportunities for species to evolve and diversify in isolation. Its species are a mix of those that have been living and evolving there for many tens of millions of years and those that arrived more recently and subsequently diversified.

Understanding where all of Madagascar's species came from (i.e., its biogeography) requires understanding Madagascar's own geologic history. One hundred and seventy million years ago, Madagascar was landlocked in the middle of the supercontinent Gondwana, sandwiched between land that would eventually become South America and Africa and land that would eventually become India, Australia, and Antarctica. Through movements of the Earth's crust, Madagascar, along with India, first split from Africa and South America and then from Australia and Antarctica, and started heading north. India eventually smashed into Asia  forming the Himalayas in the process  but Madagascar broke away from India and was marooned in the Indian Ocean. Madagascar has been on its own for the past 88 million years.

Some of Madagascar's present species are there because they "rode there" on the continents and were left on the island when it separated from India. Others arrived on the island after its split, immigrating from other places. In biogeography, these two scenarios are known as vicariance and dispersal. To understand the difference, imagine a species living on a continent, which is then split into two through tectonic action. When the continent splits, the two halves of the population are separated, and over many generations, they evolve into separate species. These species' distribution is the result of vicariance. Many different processes can cause vicariance  plate tectonics, the rise of mountain ranges, a shift in the course of a river, or just climate change that causes an unfavorable habitat to develop that ends up splitting a species' range into two. Dispersal, on the other hand, occurs when a species spreads or immigrates from one area to another. If part of a population moves to a new area, the two subpopulations may eventually evolve into separate species.

One key line of evidence for investigating biogeography is phylogenetics. We can use evidence gathered from living and fossil organisms to reconstruct their phylogeny  or evolutionary relationships. These phylogenies, combined with an understanding of the geologic history of a particular region, can help us figure out which lineages are where they are because of vicariance and which are there because of dispersal. For an example, examine the diagram below. One land mass splits sequentially into three separate islands, and then a mountain range rises on one of these islands, effectively splitting it. If a group of organisms was widespread on the original land mass and was sequentially split along with the geologic changes, we'd expect the sequence of splits in the phylogeny to match the sequence of splits in the geography (stages 1-4 in the diagram). Now imagine that additional tectonic action causes one more split. After that split, some members of species C disperse to the new island, and they evolve into a separate species (stage 5 in the diagram). The relationships of species A-D match the geographic splits, but species E's relationships do not. It is most closely related to C, but it lives on an island that split off from a distant land mass. This suggests that E must have arrived at its present location by dispersal.

An elephant bird skeleton and egg.

So what about Madagascar? Do the phylogenies of Madagascar natives and their close relatives suggest that vicariance or dispersal has been at work? There are certainly some examples of vicariance. For example, the elephant bird  a ten foot tall relative of the ostrich that went extinct several hundred years ago  was endemic to Madagascar. Phylogenetic, genetic, and fossil evidence all suggest that the elephant bird, along with the ostrich, arrived on Madagascar and India when these land masses were still connected to Australia and Antarctica via a land bridge. When India and Madagascar split, the elephant bird wound up surviving on Madagascar, while the ostrich was carried north with India (and was eventually introduced to Eurasia when India collided with this continent). The presence of the elephant bird on Madagascar can be chalked up to vicariance; it was living on Madagascar land already, when Madagascar broke off of India.

However, most of the species on Madagascar today seem to be descended from individuals that dispersed there from Africa long after Madagascar was established as a separate island. For example, phylogenetic, genetic, and anatomical evidence all suggest that lemurs split from other primates on Africa around 62 million years ago and that the ancestral lemur lineage had dispersed to Madagascar by around 54 million years ago. Once on the island, the lemur lineage diversified. Now there are at least 50 species of lemur, all endemic to Madagascar.

The evolutionary and biogeographic processes experienced by the lemurs are not unusual. Madagascar is home to many groups of endemic organisms with close within-group relationships. The simplest  or most parsimonious  explanation for this pattern is that, like the lemurs, the groups first arrived on the island by dispersal as a single lineage and then rapidly diversified. This diversification was likely spurred on by other geologic and climactic characteristics of Madagascar. The east coast of the island is lined with a mountain range  and this causes different parts of the island to get drastically different amounts of rain. Hence, the island is made of many different habitat types  from deserts to rainforests  that have shifted and changed over the past 88 million years. This likely provided many opportunities for subpopulations to become isolated and evolve traits for specializing in different niches. And that likely encouraged lineages to diversify.

Today, Madagascar is one of the most diverse places on Earth. Understanding where that diversity comes from requires understanding not just the living world, but the geologic, geographic, and climactic histories that have shaped the evolution of lineages on the island. Now, human history in the making threatens to undo tens of millions of years of evolution in just a few years of political turmoil  unless safeguards can be put in place to protect Madagascar's unique biota from the instabilities of human institutions.

Video podcast on the geologic and evolutionary history of Madagascar provided by the National Evolutionary Synthesis Center (NESCent). To learn more, visit the NESCent website.

Yoder, A. D., and Nowak, M. D. (2006). Has vicariance or dispersal been the predominant biogeographic force in Madagascar? Only time will tell. Annual Review of Ecology, Evolution, and Systematics 37:405-431.

Imagine that a frog species is widely distributed on a continent. Through tectonic action, the continent splits into two. Then each of the two halves of the continent splits again, producing four separate land masses. If this process caused the frog lineage to speciate into four new species through vicariance, what would you expect the phylogeny of the frog species to look like? Don't worry about labeling the branches of the tree; just draw the shape of the phylogeny.

Imagine that a lizard species lives on a small volcanic island. Some individuals from this population disperse to a neighboring island and evolve into two new species there (species A and B). Then a few more individuals from the original population disperse to a different island, where they evolve into species C. Then, yet another group of lizards from the original population disperses to yet another island, where they evolve into species D. The lizard species that remains on the original island is called species E. Draw the phylogeny of species A-E, labeling the tips of the phylogeny.

Yoder, A. D., and Nowak, M. D. (2006). Has vicariance or dispersal been the predominant biogeographic force in Madagascar? Only time will tell. Annual Review of Ecology, Evolution, and Systematics 37:405-431.